Visibility of blobs with a Gaussian luminance profile.

Contrast detection thresholds for circular symmetric stimuli with a Gaussian luminance profile and Gaussian temporal modulation were measured as a function of their spatial scale parameter (varied over two and a half decades) for four different presentation times. The stimulus was presented on a very large uniform background field. For sufficiently large stimuli the threshold is constant over a size variation of at least one and a half decades. When the stimulus is presented on a restricted background field, a loss of sensitivity is found at long presentation times when the stimulus size exceeds a certain fraction of the background diameter. The measurements show (i) that the visual system has a very large bandwidth and (ii) that the presence of edges or luminance transitions in the visual field can diminish the sensitivity to a stimulus considerably, even when they are rather distant. The results can be explained in a natural way with a multi-layer receptive field model.